ajax表单提交验证，浏览器兼容问题

function createXmlHttp() {
var xmlHttp;
try { // Firefox, Opera 8.0+, Safari
xmlHttp = new XMLHttpRequest();
} catch (e) {
try {// Internet Explorer
xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
} catch (e) {
try {
xmlHttp = new ActiveXObject(
"Microsoft.XMLHTTP");
} catch (e) {
}
}
}
return xmlHttp;
}

var xhr = createXmlHttp();

// 2.设置监听
xhr.onreadystatechange = function() {
if (xhr.readyState == 4) {
if(xhr.status==200){
var data=xhr.responseText;
var success="success";
var json=$.trim(data);
if(success==json){
document.getElementById("myform").submit();
}else{
alert("单位名称重复，请重新输入");
return false;
}
}
}
}
// 3.打开连接
xhr.open("POST",
"TbDweckRepeat?time="
+ new Date().getTime()+"&dwmc="+dwmc+"&UpdId="+id, false);
// 4.发送
xhr.send(null);